Arsenic Uptake and Accumulation Mechanisms in Rice Species

Crosstalk of plant growth regulators protects photosynthetic performance from arsenic damage by modulating defense systems in rice

Salicylic acid (SA) is a well-known plant growth regulator, which participates in many physiological processes of plants under normal and stressful conditions. In this study, we investigated the impact of SA supplementation on the components of ascorbate-glutathione cycle and glyoxalase system, photosynthesis and growth of rice (Oryza sativa) plants subjected to arsenic (As) stress. Plants grown with As exhibited enhanced As uptake, increased oxidative stress, and photosynthesis and growth inhibition. Application of SA promoted photosynthesis and growth in plants with or without As stress by improving plant defense systems and reducing oxidative stress through interaction with ethylene and nitric oxide (NO). SA acted as an ethylene antagonist, reducing stress ethylene formation under As stress, while NO formation was induced. This resulted in coordinated control over the antioxidant defense systems and enhanced As tolerance, protecting photosynthesis and growth from As-induced damage. The study showed that positive responses of SA in promoting photosynthesis and growth under As stress were the result of its interplay with ethylene and NO, enhanced capacity of defense systems to reduce oxidative stress. The crosstalk of SA with ethylene and NO will be useful in augmenting the performance of rice plants under As stress.

Fate of arsenic in living systems: Implications for sustainable and safe food chains

Arsenic, a group 1 carcinogen for humans, is abundant as compared to other trace elements in the environment and is present mainly in the Earth's crust and soil. The arsenic distributions in different geographical regions are dependent on their geological histories. Anthropogenic activities also contribute significantly to arsenic release into the environment. Arsenic presents several complications to humans, animals, and plants. The physiology of plants and their growth and development are affected by arsenic. Arsenic is known to cause cancer and several types of organ toxicity, such as cardiotoxicity, nephrotoxicity, and hepatotoxicity. In the environment, arsenic exists in variable forms both as inorganic and organic species. From arsenic containing compartments, plants can absorb and accumulate arsenic. Crops grown on these contaminated soils pose several-fold higher toxicity to humans compared with drinking water if arsenic enters the food chain. Information regarding arsenic transfer at different trophic levels in food chains has not been summarized until now. The present review focuses on the food chain perspective of arsenic, which affects all components of the food chain during its course. The circumstances that facilitate arsenic accumulation in flora and fauna, as components of the food chain, are outlined in this review.

Strigolactones regulate arsenate uptake, vacuolar-sequestration and antioxidant defense responses to resist arsenic toxicity in rice roots

We explored genetic evidence for strigolactones' role in rice tolerance to arsenate-stress. Comparative analyses of roots of wild-type (WT) and strigolactone-deficient mutants d10 and d17 in response to sodium arsenate (Na₂AsO₄) revealed differential growth inhibition [WT (11.28%) vs. d10 (19.76%) and d17 (18.03%)], biomass reduction [(WT (33.65%) vs. d10 (74.86%) and d17 (60.65%)] and membrane damage (WT < d10 and d17) at 250 μM Na₂AsO₄. Microscopic and biochemical analyses showed that roots of WT accumulated lower levels of arsenic and oxidative stress indicators like reactive oxygen species and malondialdehyde than those of strigolactone-deficient mutants. qRT-PCR data indicated lower expression levels of genes (OsPT1, OsPT2, OsPT4 and OsPT8) encoding phosphate-transporters in WT roots than mutant roots, explaining the decreased arsenate and phosphate uptake by WT roots. Increased levels of glutathione and OsPCS1 and OsABCC1 transcripts indicated an efficient vacuolar-sequestration of arsenic in WT roots. Furthermore, higher activities (transcript levels) of SOD (OsCuZnSOD1 and OsCuZnSOD2), APX (OsAPX1 and OsAPX2) and CAT (OsCATA) corresponded to lower oxidative damage in WT roots compared with strigolactone-mutant roots. Collectively, these results highlight that strigolactones are involved in arsenic-stress mitigation by regulating arsenate-uptake, glutathione-biosynthesis, vacuolar-sequestration of arsenic and antioxidant defense responses in rice roots.

Bioaugmentation with As-transforming bacteria improves arsenic availability and uptake by the hyperaccumulator plant Pteris vittata (L)

Inorganic arsenic (As) is a toxic and carcinogenic pollutant that has long-term impacts on environmental quality and human health. Pteris vittata plants hyperaccumulate As from soils. Soil bacteria are critical for As-uptake by P. vittata. We examined the use of taxonomically diverse soil bacteria to modulate As speciation in soil and their effect on As-uptake by P. vittata. Aqueous media inoculated with Pseudomonas putida MK800041, P. monteilii MK344656, P. plecoglossicida MK345459, Ochrobactrum intermedium MK346993 or Agrobacterium tumefaciens MK346997 resulted in the oxidation of 5-30% As(III) and a 49-79% reduction of As(V). Soil inoculated with P. monteilii increased extractable As(III) and As(V) from 0.5 and 0.09 in controls to 0.9 and 0.39 mg As kg^-1 soil dry weight, respectively. Moreover, and P. vittata plants inoculated with P. monteilii, P. plecoglossicida, O. intermedium strains, and A. tumefaciens strains MK344655, MK346994, MK346997, significantly increased As-uptake by 43, 32, 12, 18, 16, and 14%, respectively, compared to controls. The greatest As-accumulation (1.9 ± 0.04 g kg^-1 frond Dwt) and bioconcentration factor (16.3 ± 0.35) was achieved in plants inoculated with P. monteilii. Our findings indicate that the tested bacterial strains can increase As-availability in soils, thus enhancing As-accumulation by P. vittata. Novelty statement Pteris vittata, a well-known As-hyperaccumulator, has the remarkable ability to accumulate higher levels of As in their above-ground biomass. The As-tolerant bacteria-plant interactions play a significant role in bioremediation by mediating As-redox and controlling As-availability and uptake by P. vittata. Our studies indicated that most of the tested bacterial strains isolated from As-impacted soil significantly enhanced As-uptake by P. vittata. P. monteilii oxidized 20% of As(III) and reduced 50% of As(V), increased As-extraction from soils, and increased As-uptake by 43% greater compared with control. Therefore, these strains associated with P. vittata can be used in large-scale field applications to remediate As-contaminated soil.

Exogenous melatonin regulates endogenous phytohormone homeostasis and thiol-mediated detoxification in two indica rice cultivars under arsenic stress

Melatonin enhanced arsenic (As) tolerance by inhibiting As bioaccumulation, modulating the expression of As transporters and phytohormone homeostasis, leading to efficient utilization of thiol machinery for sequestration and detoxification of this toxic metalloid. The present study was aimed at investigating the influence of exogenous melatonin on the regulation of endogenous plant growth regulators and their cumulative effects on metal(loid)-binding ligands in two contrasting indica rice cultivars, viz., Khitish (arsenic sensitive) and Muktashri (arsenic tolerant) under arsenic stress. Melatonin supplementation ameliorated arsenic-induced perturbations by triggering endogenous levels of gibberellic acid and melatonin, via up-regulating the expression of key biosynthetic genes like GA3ox, TDC, SNAT and ASMT. The endogenous abscisic acid content was also enhanced upon melatonin treatment by induced expression of the key anabolic gene, NCED3 and concomitant suppression of ABA8ox1. Enhanced melatonin content induced accumulation of higher polyamines (spermidine and spermine), together with up-regulation of SPDS and SPMS in Khitish, thereby modulating stress condition. On the contrary, melatonin escalated putrescine and spermidine levels in Muktashri, via enhanced expression of ADC and SAMDC. The role of melatonin appeared to be more prominent in Khitish, as evident from better utilization of thiol components like cysteine, GSH, non-protein thiols and phytochelatins, with higher GSH/GSSG ratio, despite down-regulated expression of corresponding thiol-metabolic genes (OsMT2 and OsPCS1) to deal with arsenic toxicity. The extent of arsenic bioaccumulation, which was magnified several folds, particularly in Khitish, was decreased upon melatonin application. Overall, our observation highlighted the fact that melatonin enhanced arsenic tolerance by inhibiting arsenic bioaccumulation, via modulating the expression levels of selected arsenic transporters (OsNramp1, OsPT2, OsPT8, OsLsi1) and controlling endogenous phytohormone homeostasis, leading to efficient utilization of thiol machinery for sequestration and detoxification of this toxic metalloid.

Nutritional Evaluation and Risk Assessment of the Exposure to Essential and Toxic Elements in Dogs and Cats through the Consumption of Pelleted Dry Food: How Important Is the Quality of the Feed?

Dry feed for pets lacks specific legislation regarding maximum residue limits for inorganic elements. The aim of the present study was to determine the content of 43 inorganic elements in dog and cat feed, studying whether there were differences according to the supposed quality of the food and performing the risk assessment for health. Thirty-one and thirty packages of pelleted dry food for cats and dogs, respectively, were analyzed. After acidic microwave-assisted digestion, elements were detected and quantified by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). In general, we did not observe important differences in the content of elements according to the supposed quality of the brand. Among trace elements, selenium and manganese are above the dietary reference value. Arsenic and mercury showed the highest acute hazard indexes, which make them risk factors for the health of dogs and cats. Aluminum, uranium, antimony and vanadium contents were above the toxic reference value and showed the highest acute hazard indexes. It is necessary to improve the legislation regarding the food safety of pets, for their health and to protect the rights of consumers.

An amiRNA screen uncovers redundant CBF and ERF34/35 transcription factors that differentially regulate arsenite and cadmium responses

Arsenic stress causes rapid transcriptional responses in plants. However, transcriptional regulators of arsenic-induced gene expression in plants remain less well known. To date, forward genetic screens have proven limited for dissecting arsenic response mechanisms. We hypothesized that this may be due to the extensive genetic redundancy present in plant genomes. To overcome this limitation, we pursued a forward genetic screen for arsenite tolerance using a randomized library of plants expressing >2,000 artificial microRNAs (amiRNAs). This library was designed to knock-down diverse combinations of homologous gene family members within sub-clades of transcription factor and transporter gene families. We identified six transformant lines showing an altered response to arsenite in root growth assays. Further characterization of an amiRNA line targeting closely homologous CBF and ERF transcription factors show that the CBF1,2 and 3 transcription factors negatively regulate arsenite sensitivity. Furthermore, the ERF34 and ERF35 transcription factors are required for cadmium resistance. Generation of CRISPR lines, higher-order T-DNA mutants and gene expression analyses, further support our findings. These ERF transcription factors differentially regulate arsenite sensitivity and cadmium tolerance.

Overexpression of rice OsWNK9 promotes arsenite tolerance in transgenic Arabidopsis plants

Protein kinases are involved in the transfer of phosphate group to serine, threonine, and tyrosine residues of a target protein. With No Lysine (WNK) kinase is a member of the serine/threonine protein kinase family, which has conserved catalytic lysine (K) residue in subdomain I instead of being in subdomain II.The WNKs family members in plants are stress inducible and have been validated for their role in abiotic stress tolerance. In the present study, we have characterized Arabidopsis overexpressed lines of OsWNK9 regulated by the constitutive promoter under arsenite stress. Moreover, we have performed In silico expression analysis of OsWNK9 under nutrient deficiency and heavy metal stress. Three independent transgenic Arabidopsis (OsWNK9-OX T₁₁, T₁₂,andT₁₃) lines showed tolerance to arsenite stress compared to wild-type (WT) plants. Under arsenite stress, transgenic lines T₁₁, T₁₂ and T₁₃ showed 56.46, 57.8 and 51.66 % increased biomass respectively, as compared to WT plants. All three ArabidopsisOsWNK9-OX lines exhibited higher proline content, increased antioxidant enzyme activities and lower hydrogen peroxide levels under arsenite stress. Besides, the total antioxidant capacity in terms of DPPH (2, 2-diphenyl-1-picrylhydrazyl) free radical scavenging percentage was increased by 8-15 % in three independent OsWNK9-OX lines compared with those of WT plants. Protein-protein interaction analysis of OsWNK9 predicted interaction partners with protein kinase and oxidative stress-responsive protein. Co-expression analysis of OsWNK9 in phosphate deficiency and arsenate stress condition predicted various proteins including membrane transporter and transcription factors. Taken together, our results, for the first time, provide evidence that OsWNK9 could positively mediate arsenite stress tolerance in plants.

Main nitric oxide (NO) hallmarks to relieve arsenic stress in higher plants

Arsenic (As) is a toxic metalloid that adversely affects plant growth, and poses severe risks to human health. It induces disturbance to many physiological and metabolic pathways such as nutrient, water and redox imbalance, abnormal photosynthesis and ATP synthesis and loss of membrane integrity. Nitric oxide (NO) is a free radical molecule endogenously generated in plant cells which has signalling properties. Under As-stress, the endogenous NO metabolism is significantly affected in a clear connection with the metabolism of reactive oxygen species (ROS) triggering nitro-oxidative stress. However, the exogenous NO application provides beneficial effects under As-stress conditions which can relieve oxidative damages by stimulating the antioxidant systems, regulation of the expression of the transporter and other defence-related genes, modification of root cell wall composition or the biosynthesis of enriched sulfur compounds such phytochelatins (PCs). This review aims to provide up-to-date information on the key NO hallmarks to relieve As-stress in higher plants. Furthermore, it will be analyzed the diverse genetic engineering techniques to increase the endogenous NO content which could open new biotechnological applications, especially in crops under arsenic stress.

Hydrogen peroxide modulates activity and expression of antioxidant enzymes and protects photosynthetic activity from arsenic damage in rice (Oryza sativa L.)

We studied the role of H₂O₂ in the protection of photosynthesis from arsenic (As) damage in rice (Oryza sativa L.) by examining the antioxidant system, photosynthesis, and growth attributes. Among the As concentrations (0, 20, 30, 40 and 50 μM) tested, maximum oxidative stress and inhibition in photosynthesis and growth were found with 50 μM As. The application of 50 μM H₂O₂ resulted in alleviation of the adverse effects of 50 μM As on Pigment System (PS) II activity, photosynthesis, and growth. Hydrogen peroxide supplementation induced the activity of superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione reductase (GR) and increased reduced glutathione (GSH) content and proline metabolism. The expression of SOD and APX, PSBA and PSBB was induced in the presence of H₂O₂ to alleviate the As damage to PS II and maintain photosynthetic activity. The role of H₂O₂ as a signaling molecule is shown in the protection of photosynthetic activity in rice from As toxicity through regulation on the activity and the expression of antioxidant enzymes.

Calcium Plays a Double-Edged Role in Modulating Cadmium Uptake and Translocation in Rice

Cadmium (Cd) contamination in soils poses great risks to both agricultural production and human health. Calcium (Ca) is an essential element playing a significant role in protecting plants against Cd toxicity. However, how Ca affects Cd uptake and translocation in rice is still not fully elucidated. In this study, the regulatory role of Ca in Cd uptake and upward translocation was investigated in rice at different growth stages. Our results showed that the supplement of 5 mM Ca significantly reduced Cd uptake by rice roots, because of their competition for Ca-permeable channels as an absorption site and Ca-induced downregulation of OsNRAMP1 and OsNRAMP5. However, Ca application facilitated the upward translocation of Cd by both upregulating OsHMA2 to induce xylem loading of Cd and downregulating OsHMA3 to reduce vacuolar sequestration of Cd. Such contrary results suggested a double-edged role of Ca in regulating root Cd uptake and root-to-shoot Cd translocation in rice. Although it increased Cd content in the aboveground vegetative tissues during the whole growth period, the addition of 5 mM Ca eventually decreased Cd content in rice grains at the ripening stage. All these results suggest that Ca-based amendments possess great potential for the production of low-Cd rice grains.

Major As species, lipid peroxidation and protein carbonylation in rice plants exposed to increasing As(V) concentrations

Arsenic (As) uptake by plants is mainly carried out as arsenate (As(V)), whose chemical analogy with phosphate is largely responsible for its elevated toxicity. Arsenate is known to stimulate reactive oxygen species (ROS) formation in plants that provoke oxidative stress. This manuscript reports the results of a hydroponics study using rice (Oryza sativa L.) seedlings as a test plant, where the effects of increasing arsenate concentrations (0–10 mg L⁻¹) on both lipid and protein oxidation, as well as As accumulation and speciation in plant roots and shoots were examined. Plant yield was negatively affected by increasing As concentration. Accumulation in plant roots was higher than in shoots at low arsenate doses (0.5–2.5 mg L⁻¹), while root to shoot transport was drastically enhanced at the highest doses (5 and 10 mg L⁻¹). Moreover, As(V) was the dominating species in the shoots and As(III) in the roots. Rice leaves in the 10 mg As L⁻¹ treatment showed the highest lipid peroxidation damage (malondialdehyde concentration), whilst protein oxidation was not remarkably influenced by As dose. Lipid peroxidation seems to be therefore conditioned by As accumulation in rice plants, particularly by the presence of high As(V) concentrations in the aerial part of the plants as a consequence of unregulated translocation from roots to shoots above a threshold concentration (1.25–2.5 mg L⁻¹) in the growing media. These results provide relevant information regarding As(V) toxic concentrations for rice plants, highlight the importance of major As species analysis in plant tissues regarding As toxicity and contribute to better understand plants response to elevated As concentrations in the growing media.

Reactive Oxygen Species (ROS) Metabolism and Nitric Oxide (NO) Content in Roots and Shoots of Rice (Oryza sativa L.) Plants under Arsenic-Induced Stress

Arsenic (As) is a highly toxic metalloid for all forms of life including plants. Rice is the main food source for different countries worldwide, although it can take up high amounts of As in comparison with other crops, showing toxic profiles such as decreases in plant growth and yield. The induction of oxidative stress is the main process underlying arsenic toxicity in plants, including rice, due to an alteration of the reactive oxygen species (ROS) metabolism. The aim of this work was to gain better knowledge on how the ROS metabolism and its interaction with nitric oxide (NO) operate under As stress conditions in rice plants. Thus, physiological and ROS-related biochemical parameters in roots and shoots from rice (Oryza sativa L.) were studied under 50 μM arsenate (AsV) stress, and the involvement of the main antioxidative systems and NO in the response of plants to those conditions was investigated. A decrease of 51% in root length and 27% in plant biomass was observed with 50 μM AsV treatment, as compared to control plants. The results of the activity of superoxide dismutase (SOD) isozymes, catalase, peroxidase (POD: total and isoenzymatic), and the enzymes of the ascorbate–glutathione cycle, besides the ascorbate and glutathione contents, showed that As accumulation provoked an overall significant increase of most of them, but with different profiles depending on the plant organ, either root or shoot. Among the seven identified POD isozymes, the induction of the POD-3 in shoots under As stress could help to maintain the hydrogen peroxide (H2O2) redox homeostasis and compensate the loss of the ascorbate peroxidase (APX) activity in both roots and shoots. Lipid peroxidation was slightly increased in roots and shoots from As-treated plants. The H2O2 and NO contents were enhanced in roots and shoots against arsenic stress. In spite of the increase of most antioxidative systems, a mild oxidative stress situation appears to be consolidated overall, since the growth parameters and those from the oxidative damage could not be totally counteracted. In these conditions, the higher levels of H2O2 and NO suggest that signaling events are simultaneously occurring in the whole plant.

Selenite Foliar Application Alleviates Arsenic Uptake, Accumulation, Migration and Increases Photosynthesis of Different Upland Rice Varieties

This study investigates how arsenic (As) uptake, accumulation, and migration responds to selenium (Se) foliar application (0–5.0 mg × kg⁻¹). Rice varieties known to accumulate low (DOURADOAGULHA) and high (SINALOAA68) concentrations of arsenic were chosen to grow on soil with different As concentrations (20.1, 65.2, 83.9 mg × kg⁻¹). The results showed that Se of 1.0 mg × L⁻¹ significantly alleviated As stress on upland rice grown on the As-contaminated soil. Under light (65.2 mg × kg⁻¹) and moderate (83.9 mg × kg⁻¹) As concentration treatments, the biomass of upland rice was increased by 23.15% and 36.46% for DOURADOAGULHA, and 46.3% and 54.9% for SINALOAA68. However, the high Se dose (5.0 mg × kg⁻¹) had no significant effect on biomass and heights of upland rice compared to plants where no Se was added. Se significantly decreased As contents in stems and leaves and had different effects on As transfer coefficients for the two rice varieties: when grown on soil with low and moderate As concentrations, Se could reduce the transfer coefficient from stems to leaves, but when grown on the high As soils, this was not the case. The chlorophyll content in plants grown in soil with the moderate concentration of As could be improved by 27.4%–55.3% compared with no Se treatment. Under different As stress, the Se foliar application increased the net photosynthesis, stomatal conductance, and transpiration rate, which meant that Se could enhance the photosynthesis of rice. The intercellular CO₂ concentration variation implied that the stomatal or non-stomatal limitations could both occur for different rice varieties under different Se application doses. In conclusion, under moderate As stress, foliar application of Se (1.0 mg × L⁻¹) is recommend to overcome plant damage and As accumulation.

Cadmium Uptake by Wheat (Triticum aestivum L.): An Overview

Cadmium is a toxic heavy metal that may be detected in soils and plants. Wheat, as a food consumed by 60% of the world’s population, may uptake a high quantity of Cd through its roots and translocate Cd to the shoots and grains thus posing risks to human health. Therefore, we tried to explore the journey of Cd in wheat via a review of several papers. Cadmium may reach the root cells by some transporters (such as zinc-regulated transporter/iron-regulated transporter-like protein, low-affinity calcium transporters, and natural resistance-associated macrophages), and some cation channels or Cd chelates via yellow stripe 1-like proteins. In addition, some of the effective factors regarding Cd uptake into wheat, such as pH, organic matter, cation exchange capacity (CEC), Fe and Mn oxide content, and soil texture (clay content), were investigated in this paper. Increasing Fe and Mn oxide content and clay minerals may decrease the Cd uptake by plants, whereas reducing pH and CEC may increase it. In addition, the feasibility of methods to diminish Cd accumulation in wheat was studied. Amongst agronomic approaches for decreasing the uptake of Cd by wheat, using organic amendments is most effective. Using biochar might reduce the Cd accumulation in wheat grains by up to 97.8%.